Provide heartbeat update subscription mechanism

[evan-gray]

Expected Behavior

I would like to reuse the p2p component to be able to spy on guardian heartbeats. There should be some mechanism to receive an update via a channel akin to observations and VAAs.

Current Behavior

A NewGuardianSetState is passed into p2p and updated directly with heartbeats, without an opportunity to receive the heartbeat.

Possible Solution

The GuardianSetState could take an update channel as an argument and publish to the channel whenever the set is updated (SetHeartbeat)

Context (Environment)

Building an indexer for the wormhole ecosystem.
https://github.com/wormhole-foundation/wormhole-explorer/blob/main/fly/main.go#L110